The Collaborative International Dictionary of English v.0.48:

Lyceum \Ly*ce"um\, n.; pl. E. Lyceums, L. Lycea. [L. lyceum,
   Gr. ?, so named after the neighboring temple of ? ? Apollo
   the wolf slayer, prob. fr. ? belonging to a wolf, fr ? wolf.
   See Wolf.]
   1. A place of exercise with covered walks, in the suburbs of
      Athens, where Aristotle taught philosophy.
      [1913 Webster]

   2. A house or apartment appropriated to instruction by
      lectures or disquisitions.
      [1913 Webster]

   3. A higher school, in Europe, which prepares youths for the
      university.
      [1913 Webster]

   4. An association for debate and literary improvement.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lyceum
    n 1: a school for students intermediate between elementary
         school and college; usually grades 9 to 12 [syn: secondary
         school, lyceum, lycee, Gymnasium, middle school]
    2: a public hall for lectures and concerts